In theory, your deals can remain open for as long as you wish. Overnight Financing and/or Rollovers are applied to CFDs in order to keep the deals open until you decide to manually close them or until a closing limit is reached (Stop Loss, Take Profit, Negative Balance Protection, etc.).
However, note that deals on share CFDs will be automatically close if a Corporate Event related to the relevant share occurs. Corporate Events, such as Splits, Delistings, Spin-Offs, etc. are not always prescheduled. They can be announced even one week before they occur and are not always recurrent.
For additional information regarding Overnight Financing, Rollovers and Corporate Events, kindly visit the Trading Condition page.